Thin film interference filters are widely used in systems for optical measurement and analysis, such as Raman spectroscopy and fluorescence imaging, for example. Thin film interference filters, including optical edge filters, notch filters, and laser line filters (LLFs) are advantageously used in such systems to transmit light having specific wavelength bands and to block unwanted light that could otherwise constitute or generate spurious optical signals and swamp the signals to be detected and analyzed. Dichroic beam splitters utilize interference filter effects to reflect certain wavelengths or ranges of wavelengths and transmit other wavelengths or ranges of wavelengths. Failure or poor performance of such filters compromises the performance of systems in which they are used. Conventional design approaches for optical instruments that utilize thin-film filters are often constrained by inherent characteristics of these filters and long-standing practices for how these filters are designed and used.
As an example of one type of system that relies heavily on thin-film filters and benefits from high performance filter design, the simplified schematic diagram of FIG. 1 shows one type of imaging apparatus that is used for analysis of spectral characteristics of a sample. A fluorescence microscopy system 10 has a light source 12 with an illumination lens L2 that directs a beam of excitation energy, within a specific wavelength range, toward a sample 20 for analysis. Optical fluorescence occurs when absorption of light of the excitation wavelength(s) causes emission of light at one or more longer wavelengths. A succession of filters 22, 24, and a beam splitter 26 are used to isolate the different wavelength bands of light from sample 20 through lens L1 to their appropriate paths, through lens L3, so that the desired emitted signal content, which can be at orders of magnitude lower than the excitation energy, can be properly sensed at a detector 30, such as a camera or charge-coupled device (CCD).
The detection problem becomes more complex when there are multiple emission wavelengths, such as where multiple fluorophores are used within the sample or multiple lines are detected in Raman spectroscopy. The simplified schematic diagram of FIG. 2A shows a fluorescence microscopy system 50 that uses a beam splitter 52 as an image splitter to provide an image of a first wavelength band to a first detector 60 and an image of a second wavelength band to a second detector 62. Beam splitter 52 is at a 45 degree angle with respect to the propagation direction of incident light. Additional filters 56 and 58 are used to help further isolate the image content according to wavelength. Lenses L1 and L2 function as objective and illumination lenses, respectively. Lenses L5 and L6 serve to direct the image-bearing light to detectors 60 and 62.
The simplified schematic diagram of FIG. 2B shows a fluorescence microscopy system 70 that obtains two images and provides them on a single detector 30. Image-bearing light is directed from a lens L7 to a beam splitter 32 that transmits one wavelength band toward a mirror 34 and reflects the complementary wavelength band. The transmitted wavelength band reflects from mirror 34 and is directed toward a movable mirror 36, through emission filter 58, and through a lens L8 toward detector 30, forming a first image. Light of the complementary wavelength band that had been reflected from beam splitter 32 is redirected by movable mirror 36 through emission filter 56. This light also goes through lens L8 and forms a second image on detector 30.
The arrangements of components shown in FIGS. 2A and 2B provide workable solutions for separating images of first and second wavelength bands, but have a number of shortcomings. The requirement for two detectors 60 and 62 in the FIG. 2A embodiment adds cost and complexity, significantly increasing the size of the microscopy apparatus. Detectors 60 and 62 are orthogonal to each other in conventional apparatus; the proper positioning of detectors 60 and 62 requires a relatively bulky mounting arrangement. Both of the FIG. 2A and FIG. 2B embodiments add a significant number of components, increasing the cost and overall weight and bulk of the microscopy system.
Dichroic filters and, more broadly, thin-film interference filters in general are conventionally designed to provide desired behavior for light that is incident over a small range of angles, typically angles that are near normal incidence. With many thin-film designs, light behaves well at the design angle of incidence; but this behavior can degrade rapidly as the incident light varies further from the design angle of incidence. Conventional thin-film filter designs often exhibit high sensitivity to angle of incidence (AOI) and cone half angle (CHA). For this reason, conventional design practice avoids directing incident light that is at high incident angles (relative to normal) onto dichroic and other types of thin-film surfaces. This practice sets a number of constraints on how components are arranged for separating image-bearing light according to its spectral characteristics, often making it difficult to package optical components for image splitting in a compact configuration.
Thus, it can be seen that there is a need for improved dichroic image splitter approaches for use in spectroscopy, fluorescence microscopy, and other applications.